1. Field of the Invention
The present invention relates to an axle driving apparatus including a housing and a hydrostatic transmission (HST) contained in the housing. More particularly, it relates to a cooling system for oil in the housing, an arrangement of an oil reservoir for regulating the volume of oil in the housing, and a cooling system for cooling oil in the reservoir in combination with an oil reservoir.
2. Related Art
An axle driving apparatus generally includes an HST having a hydraulic pump and a hydraulic motor fluidly connected with each other, a differential gear unit, and a transmission between a motor shaft of the HST and the differential gear unit. The HST is immersed in an oil sump formed in a housing. Such an axle driving apparatus is generally air-cooled by a cooling fan provided on an input shaft of the hydraulic pump projecting outward from the housing and by fins formed on an outer wall of the housing.
As described in Japanese Laid-Open Gazette Hei. 3-159822, a known axle driving apparatus includes a housing, an interior thereof being divided into first and second chambers. The first chamber contains a hydraulic pump and a hydraulic motor of an HST, and the second chamber contains a differential gear unit and a gear train forming the transmission. An oil reservoir is disposed above the first chamber.
When such a conventional axle driving apparatus utilizes the above mentioned air-cooling system having a cooling fan, the oil located in the housing is not sufficiently cooled by air-cooling the wall of the housing. Thus, the efficiency and life of the HST are reduced.
Known oil reservoirs, as shown by Japanese Laid-Open Gazette Hei. 3-159822, project upwardly from the housing whereby a large vertical space is required for mounting the axle driving apparatus on a vehicle. Also, reservoirs which are disposed so high are generally juxtaposed with an input shaft of the HST which projects vertically upward from the housing. Thus, the reservoir is removed from the air-cooled area exposed to the cooling wind of the cooling fan. Thus, oil in the reservoir cannot be sufficiently cooled by the cooling fan.
When the oil is not cooled sufficiently, the temperature of the oil increases and the HST cannot operate smoothly.